Error : 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1) in Email Security Cloud

Products

• Email Security.cloud

Issue/Introduction

Track and Trace displays an error message "553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)" when an email is sent to external recipient.

Environment

Email Security.cloud

Resolution

• This error indicates that the recipient mail server (SMTP server) requires to be properly authenticated (logged in with their email address and password) before it will accept your emails for receiving.
• Please contact the recipient's admin to investigate further and fix this issue.

Additional Information

More info can be found in the Microsoft article.